1. Technical Field
The present invention relates to starters which have a pinion tube spline-fitted on an output shaft and are configured to shift the pinion tube relative to the output shaft in a direction away from a motor and thereby bring a pinion supported on a non-motor-side end portion of the pinion tube into mesh with a ring gear of an engine.
2. Description of Related Art
There is known, for example from Japanese Patent Application Publication No. 2006-177168 (to be simply referred to as Patent Document 1 hereinafter), a starter that has a cantilever structure.
Specifically, as shown in FIG. 5, the starter includes: an output shaft 100 configured to be driven by a motor (not shown); a pinion tube 120 fitted on the output shaft 100 via a pair of bearings 110; a one-way roller clutch 130 configured to transmit rotation of the output shaft 100 to the pinion tube 120; a pinion 140 that is straight-spline-fitted on a non-motor-side end portion (i.e., a left end portion in FIG. 5) of the pinion tube 120; and a housing 160 that supports the pinion tube 120 via a bearing 150 axially positioned between the clutch 130 and the pinion 140. Further, the starter is configured so that with operation of an electromagnetic switch (not shown), the pinion tube 120 and the clutch 130 are together shifted relative to the output shaft 100 in the axial direction away from the motor (i.e., in the leftward direction in FIG. 5), thereby bringing the pinion 140 fitted on the pinion tube 120 into mesh with a ring gear (not shown) of an engine.
With the above configuration, however, the pinion tube 120, the clutch 130 and the pinion 140 together make up a moving body that is shifted by operation of the electromagnetic switch in the axial direction away from the motor for bringing the pinion 140 into mesh with the ring gear of the engine. Consequently, the mass of the moving body may be too large to minimize the size of the electromagnetic switch that is configured to create a magnetic attraction for shifting the moving body.
On the other hand, there is known, for example from Japanese Patent Application Publication No. 2007-146759 (to be simply referred to as Patent Document 2 hereinafter), another starter that also has a cantilever structure.
Specifically, as shown in FIG. 6, the starter includes a pinion shaft 170 that is helical-spline-fitted to the inner periphery of an inner 131 of the clutch 130 so as to be axially movable relative to the clutch 130. Further, on a non-motor-side end portion (i.e., a left end portion in FIG. 6) of the pinion shaft 170, there is mounted the pinion 140.
With the above configuration, only the pinion shaft 170 and the pinion 140 together make up a moving body that is shifted by operation of the electromagnetic switch in the axial direction away from the motor for bringing the pinion 140 into mesh with the ring gear of the engine. That is, the clutch 130 is kept axially unmoved and thus not included in the moving body. Consequently, the mass of the moving body can be reduced in comparison with that in the starter disclosed in Patent Document 1, thereby making it possible to minimize the size of the electromagnetic switch.
However, in the starter disclosed in Patent Document 2, the inner 131 of the clutch 130 has female helical splines formed on an inner periphery thereof, while the pinion shaft 170 has male helical splines formed on an outer periphery thereof for meshing with the female helical splines. Therefore, on the radially inside of the inner 131, there exists a radial clearance between the inner 131 and the pinion shaft 170 that are helical-spline-fitted to each other. On the other hand, on the radially outside of the inner 131, there exist both a radial clearance between the inner 131 and rollers 133 of the clutch 130 and a radial clearance between the rollers 133 and an outer 132 of the clutch 130. That is, the radial clearances exist on both the radially inside and outside of the inner 131. Consequently, the pinion shaft 170 may be considerably inclined due to the radial clearances, causing wear of other components including the bearing 150 for supporting the pinion shaft 170 and gears of a speed reducer (not shown) for reducing the rotational speed of the motor. As a result, it may be difficult to secure high durability (or a long service life) of the starter.